1. Field of the Invention
The present invention relates to a tubular body and a method for producing the tubular body.
2. Description of the Related Art
In many nuclear reactors today, nuclear fuel is contained in a sealed metal tube called a “cladding tube”, and this cladding tube is generally made of a zirconium alloy or a steel alloy. The cladding tube is designed so that any radioactive gas and solid fission product are surely held in the tube and not released to a coolant during normal operation or during a conceivable accident. If the cladding tube is damaged, heat, hydrogen and ultimately the fission product can be released to the coolant.
Problems with conventional cladding tube are known. For example, the metal cladding tube is relatively soft, and may come into contact with a fragment which sometimes flows into a cooling system and contacts with fuel, so that the metal cladding tube wears or corrodes. As a result, such wear or corrosion is likely to lead to damage of a boundary wall of a metal containment, and consequently to release of the fission product into the coolant. Further, the metal cladding tube exothermicly react with hot water of 1,000° C. or higher to result in the addition of further heat to fission product decay heat generated by the nuclear fuel. This additional heat from the cladding further amplifies the seriousness and duration period of an accident, for example, as occurred at Three Mile Island.
JP-A-2008-501977 describes a multilayer ceramic tube (cladding tube) including an inner layer of monolithic SiC; an intermediate layer as a composite material in which SiC fibers are surrounded by a SiC matrix; and an outer layer of SiC. The contents of JP-A-2008-501977 are incorporated herein by reference in their entirety.
However, in such a cladding tube composed of SiC as a whole, the monolithic SiC layer is formed by Chemical Vapor Deposition (CVD) or the like, and all of the inner layer, the intermediate layer and the outer layer have high rigidity, so that particularly, strain generated by an immediate change in temperature which occurs at the start of an operation can develop into a crack penetrating front to back sides of the cladding tube.
Further, in order to prevent such a problem, the JP-A-2008-501977 proposes to provide each fiber with double coatings including an inside pyrolytic carbon sublayer for preventing the development of the crack and an outside SiC sublayer for protecting carbon from an oxidizing environment. However, in order to provide each fiber with the double coatings, film forming has to be performed on a fiber surface in a state where each fiber is dispersed. If each fiber can not be sufficiently dispersed, it can be coated with only either one of the above-described sublayers. Accordingly, the SiC fiber and the SiC matrix come into contact with each other to cause that the pyrolytic carbon layer would not sufficiently function.
Further, in JP-A-2008-501977, the SiC matrix is formed by two processes of providing the SiC fiber surface with the coatings and multistage treatment in which CVD or polymer penetration/pyrolysis are repeated. Such a production method requires repetition of treatment many times, resulting in complicated processes.